Frame-supported resistor grids of the type generally used prior to our invention disclosed hereinafter are shown in Kirilloff U.S. Pat. Nos. 4,100,526 and 4,651,124. The resistor strip or ribbon is fan-folded back and forth and held in place between side members by studs affixed to certain of the folds or loops, which studs are received by holes in the side members, if they are of insulating material, or, where the side members are metal, by bushings of insulating material fixed in those members.
The space allotted to dynamic braking resistors in diesel-electric locomotives is limited. The resistor units are mounted vertically in banks, face to face, adjacent ventilating fans which draw air through the bank. A loop of sagging ribbon may make contact with an adjoining loop, thus shorting part of the resistor strip. Prior to our invention to be described hereinafter, the length of ribbon between its folds or loops had to be limited to avoid sagging, and the fan-folded ribbon was therefore arranged in a number of rows or columns, in the same plane, with separator plates therebetween carrying bushings of insulating material. One attempted solution to this problem is disclosed in Harkness U.S. Pat. No. 4,651,125, issued Mar. 17, 1987. His resistor is not fan-folded strip but comprises a series of grid members preformed from metal strip with cylindrical legs at each end which fit into holes on insulating side panels. The grid members are doubled back on each other and welded or otherwise joined to provide a serpentine path from one end to the other. The members are stiffened by longitudinal ribs and can be made long enough that only two columns of grid members fill a frame.
When several resistors are assembled in banks, the cooling effect of the induced air is greater for the portions of the ribbon between loops than for the looped ends. Resistor grids commercially available prior to our invention comprised as many as eight columns or rows of fan-folded or otherwise reflexed resistor ribbon, as illustrated in the Kirilloff patents above mentioned.